Physicochemical structure and capacitive properties of activated oxidized bamboo-based porous activated carbon

Biomass-based porous activated carbon is a carbon-based material with well-developed pore structure, good adsorption, and capacitive properties. To achieve the goals of peak carbon dioxide emission and carbon neutrality, biomass-based porous activated carbon is widely used in the fields of electrochemical energy storage, capacitive deionization, and wastewater treatment owing to its low cost, environmental friendliness, and cleanness. Biomass-based porous activated carbon is one of the commonly used electrode materials for supercapacitors, whose electrochemical performance is mainly influenced by its physicochemical structure. To address the problems of high energy consumption and difficult regulation of the performance of porous activated carbon electrode materials, some research has proposed the activation and oxidation gradient heat treatment technology. This technology adopts a chemical-physical co-activation synergistic control process and utilizes the waste heat of pyrolysis activation, which can reduce the consumption of chemical activator and reaction energy and alleviate environmental pollution. This technology can attain the dual objectives of efficient and clean conversion of agricultural and forestry waste and high-value utilization of porous activated carbon. In this study, porous activated carbon was prepared by using waste bamboo as the carbon source and KHCO₃ as the activator for medium- to high-temperature (500-800 ℃) activation and low-temperature (200-350 ℃) air oxidation using air for synergistic regulation. The study also investigated the physicochemical structure and electrochemical properties of porous activated carbon under the synergistic effect of different activation and oxidation temperatures. The results showed that porous activated carbon activated at 600 ℃ was oxidized by low-temperature air at 350 ℃ (PAC-600-350), and the specific surface area was increased from 154.361 to 264.235 m²/g, indicating that O₂ in the air activates the expansion of pores. An increase of oxidation temperature from 200 ℃ to 350 ℃ elevated the elemental oxygen content in the porous activated carbon and the content of surface oxygen-containing groups (such as -C=O-O and -C-OH), enhanced the wettability, and improved the defect structure and pore structure. Moreover, the porous activated carbon after activation at medium to low temperatures (600 ℃ and 700 ℃) is more sensitive and susceptible to oxidation by low temperature air. Among different activation temperatures from 500 to 800 ℃, the PAC-600-350 porous activated carbon had the highest oxygen content (25.54%) and degree of defects (2.53) at the activation temperature of 600 ℃; while the pore structure of PAC-800-350 is the most developed at the activation temperature of 800 ℃, with a specific surface area as high as 1096.18 m²/g. In addition, PAC-800-350 is less affected by the oxidation of low temperature air and its surface oxygen-containing groups change to a lesser extent. In the three-electrode test, the specific capacitance of PAC-600-350 at a current density of 1 A/g was 215.29 F/g, which is 1.47 times that of non-air oxidized porous activated carbon. At the current density of 5 A/g, PAC-600-350 exhibited higher cycling stability with a capacitance retention rate of 93.51% after 5000 cycles of charging and discharging. In the two-electrode test, the symmetrical supercapacitor with PAC-600-350 as the working electrode showed good electrochemical performance, with an energy density of 9.06 Wh/kg at a power density of 215 W/kg. Overall, PAC-600-350 porous activated carbon is considered to have potential for practical application, this study can provide a reference for high value utilization of agroforestry wastes.